A organic light-emitting device, which is a self light-emitting device, has various advantages such as a wide viewing angle, a rapid response speed, a low driving voltage, or the like, and is currently applied as a next generation flat panel display.
Generally, a basic configuration of the organic light-emitting device has a multilayer thin film structure in which a hole transport layer, a light-emitting layer, and electron transport layer that are made of organic compounds are multilayered between an anode and a cathode. The organic light-emitting device uses a principle that when electricity is applied between both electrodes, electrons are injected from the cathode and holes are injected from the anode, such that the electrons are re-combined with holes in the light emitting layer and light is emitted while an excited state energy level is decreased to a ground state energy level.
Since Eastman Kodak Corp. (US) announced that they developed a device using aromatic diamine and Alq3 as a material for forming a light-emitting layer in 1987 (Appl. Phys. Lett. p913, (1987)), research into a technology of improving performance of a phosphorescent light-emitting material such as a naphthyldiamine based light-emitting material (U.S. Pat. No. 6,549,345), a fluorescent light-emitting material such as anthracene based material (U.S. Pat. No. 6,803,121), and a light-emitting material using an iridium complex having higher efficiency ((U.S. Pat. No. 6,858,327) has been conducted. Actually, this phosphorescent light emitting material is currently applied to a small size flat panel display such as a mobile phone, or the like.
Meanwhile, in order to maximize energy transfer efficiency, a host material is generally used in the phosphorescent light emitting material. For example, as a low molecular weight phosphorescent host, there are 4,4-N,N-dicarbazole-biphenyl (CBP) and 1,3-bis(9-carbazolyl)benzene (mCP), and the like (Journal of Materials Chemistry (2003) 13, 2157-2163; Journal of Materials Chemistry (2005) 15, 2304-2315), and as a high molecular weight phosphorescent host, there are poly(N-vinylcarbazole)(PVK), and the like.
Recently, an organic light-emitting device having a mono-layer structure and a multi-layer structure using polyvinylcarbazole, which is a polymer, rather than CBF or mCP as the host in the phosphorescent device is reported by C.W. Tang Group (J. Appl. Phys., Vol. 92, No. 7, 3447).
However, in the light-emitting device, disadvantages such as degradation of the device by heat generated by the driving for a long time, or the like, may be generated. Particularly, since a material such as mCP has a low glass transition temperature Tg, in the case in which a device made using this material as a material for a thin film is driven, the device may be easily crystallized, such that color purity may be changed by heat and a life span of the device may be decreased.
Therefore, in developing a material for organic light emitting diode (OLED) device having a long life span and high efficiency, band-gap energy of the host material and heat resistance of the material that determine the color purity and efficiency are important variables. Particularly, in developing a blue phosphorescent host material, effects of the band-gap energy of the host material and heat resistance of the material are significantly large. Researches into a technology of improving efficiency and color purity through substitution at 3 and 6 positions of the carbazole have been conducted, but the expected effect or more was not shown.
The present inventors have continuously studied in order to solve this problem and thus developed a host material capable of implementing excellent color purity and high efficiency by introducing a phosphorus compound at 3 or 6 positions of carbazole compounds to increase band-gap energy between a highest unoccupied molecular orbital (HOMO) energy level and a lowest unoccupied molecular orbital (LUMO) energy level of the host material.
Particularly, a novel host material having wide band-gap energy as well as heat resistance was obtained by synthesizing a host material in which phosphaphenanthrene based derivatives that is a phosphorus compound and has excellent heat resistance and carbazole based derivatives are bonded with each other, and a configuration of the organic light-emitting device was successfully progressed.